It is well known that by measuring the sound speed (αmix) of a fluid flowing in a pipe, various parameters of the fluid may be determined, such as is described in U.S. Pat. No. 4,080,837, entitled “Sonic Measurement of Flow Rate and Water Content of Oil-Water Streams,” to Alexander et al.; U.S. Pat. No. 5,115,670, entitled “Measurement of Fluid Properties of Two-Phase Fluids Using an Ultrasonic Meter,” to Shen; and U.S. Pat. No. 4,114,439, entitled “Apparatus for Ultrasonically Measuring Physical Parameters of Flowing Media,” to Fick. Such techniques utilize a pair of acoustic transmitters/receivers (transceivers) to generate a sound signal and to measure the time it takes for the sound signal to travel between the transceivers. This is also known as a “sing-around” or “transit time” method. However, such techniques have a variety of drawbacks such as requiring precise control of the acoustic source, difficulties with inhomogeneous multiphase flows, and costly and/or complex to implement via electronics.
To elaborate, these techniques use ultrasonic acoustic signals as the sound signal measured, which are high frequency, short wavelength signals (i.e., wavelengths that are short compared to the diameter of the pipe). Typical ultrasonic devices operate near 200 k Hz, which corresponds to a wavelength of about 0.3 inches in water. In general, to allow for signal propagation through the fluid in an unimpeded and thus interpretable manner, the fluid should be homogeneous down to scale lengths of several times smaller than the acoustic signal wavelength. Thus, the criterion for homogeneity of the fluid becomes increasingly stricter with shorter wavelength signals. Consequently, inhomogeneities in the fluid, such as bubbles, gas, dirt, sand, slugs, stratification, globules of liquid, and the like, will reflect or scatter the transmitted ultrasonic signal. Such reflection and scattering inhibit the ability of ultrasonic instruments to determine the propagation velocity. For this reason, the application of ultrasonic flow meters has been limited primarily to well mixed flows.
Gamma-densitometers are widely used in the art for performing density measurements of fluids within pipes. These devices utilize a nuclear source to expose the fluids to a gamma radiation beam and measure density based on gamma beam absorption. The primary drawbacks of this type of density meter are the environmental and safety issues associated with the nuclear sources.
Another prior art method of determining the density of a fluid within a pipe is through the use of Coriolis meter. A Coriolis meter measures mass flow and density as the primary measurements by tracking the natural frequency of a vibrating pipe filled with the fluid. These devices require a vibration source, among other elements, which make Coriolis meters mechanically complex and relatively expensive to install and maintain.
As well as determining density, it is often useful in a production environment to determine the phase fraction of components flowing within the pipe. Flow meters for determining phase fractions are known in the art. See, e.g., U.S. Pat. No. 6,354,147, entitled “Fluid Parameter Measurement in Pipes Using Acoustic Pressures,” issued Mar. 12, 2002, which is incorporated by reference herein in its entirety. In this patent, a spatial array of pressure sensors, preferably fiber optic sensors, are coupled to the outside of the pipe. These sensors measure the speed that sound waves travel through the fluid by sensing the acoustic perturbations caused by naturally occurring sound waves in the fluid in the pipe. Because of the relationship between the fluid mixture sound speed and the sound speed of the components, the phase fractions of the fluid can be solved for. Moreover, if the density of the fluid can be determined, the phase fractions for a three phase fluid can be directly solved for. As typical pipelines in the oil and gas industry contain more than two phases, i.e. water, gas and oil, measuring the phase fraction of a three phase fluid mixture would be desirable.
Thus, it is desirable to provide an apparatus capable of not only measuring the density of a fluid but also the phase fraction of a multiphase fluid.